Among aromatic compounds, xylenes are very important compounds as starting materials for producing terephthalic acid, isophthalic acid, orthophthalic acid and so on, which are raw materials of polyester. While the xylenes are produced by, e.g., transalkylation, disproportionation reaction and so on of toluene, the product involves structural isomers of p-xylene, o-xylene and m-xylene. Terephthalic acid, which is obtained by oxidation of p-xylene, is used as a main raw material of polyethylene terephthalate; phthalic anhydride, which is obtained from o-xylene, is used as a raw material of plasticizers and so on; and isophthalic acid, which is obtained from m-xylene, is used as a main raw material of unsaturated polyesters and so on, respectively. Accordingly, there is a need for a method for separating these structural isomers from the product in an efficient manner.
However, there is little difference among the boiling points of p-xylene (with a boiling point of 138° C.), o-xylene (with a boiling point of 144° C.) and m-xylene (with a boiling point of 139° C.). This makes it difficult to separate these isomers by means of a normal distillation method. In response, there are methods for separating these isomers, such as a crystallization separation method where xylene mixture containing p-, o- and m-isomers is precisely distilled, which is then subjected to cooling crystallization to separate p-xylene with a high melting point, and a method for adsorbing and separating p-xylene using a zeolite-based adsorbent having a molecular sieving effect.
Such a method, where p-xylene is selectively separated by means of crystallization separation, is problematic in that the xylene mixture containing structural isomers must be subjected to precise distillation and then cooling crystallization, which results in a multi-stage, and thus more complicated process, and in that the precise distillation and cooling crystallization process lead to an increased production cost, etc. Accordingly, in place of this method, the adsorption and separation method has been most widely used. This method is of the type where p-xylene with stronger adsorbability than the other isomers is adsorbed and separated from the other isomers, while the xylene mixture as the raw material passes through the adsorption tower which is loaded with an adsorbent. Then, p-xylene is extracted from the system using a desorption agent. After the desorption, the p-xylene is separated from the desorption liquid through distillation. Practical processes include the PAREX process by UOP, AROMAX process by Toray and so on. This adsorption and separation method provides a high yield and high purity of p-xylene relative to the other separation methods. However, this method requires repeating adsorption and desorption sequentially using an adsorption tower with a pseudo-moving bed having 10 to 20-odd stages, and separately separating and removing the desorption agent for removing p-xylene from the adsorbent. As such, this method has offered by no means sufficient operating efficiency for the production of high purity p-xylene.
In contrast to this inefficient process, some attempts have been made by those skilled in the art to drastically improve the production efficiency of paraxylene. Specific examples include a method for producing paraxylene by selective methylation of toluene and so on. In this case, the methylation of toluene includes the production of paraxylene/benzene by disproportionation reaction of toluene itself. For example, Patent Document 1 listed below discloses a zeolite bound zeolite catalyst that comprises a first zeolite crystal having catalytic activity and a second zeolite crystal having a molecular sieving effect. However, in the zeolite bound zeolite catalyst disclosed in Patent Document 1, the second zeolite crystal having the molecular sieving effect forms a continuous phase matrix or bridge, and hence the proportion of the first zeolite crystal having the catalytic activity occupied in the zeolite bound zeolite catalyst becomes small, which results in decreased catalytic activity. In addition to this, if the second zeolite crystal having the molecular sieving effect forms a continuous phase matrix, the permeation resistance of a selected molecule becomes too large, which tends to decrease the molecular sieving effect. Moreover, since the second zeolite crystal serves as a binder (carrier) without the use of any binder (carrier) for shape retention, a zeolite bound zeolite catalyst with the first zeolite crystal aggregated by the second zeolite crystal, or a clumped zeolite bound zeolite catalyst will be obtained. It is considered that said aggregated or clumped catalyst requires shaping or sizing in use. In this case, however, the second zeolite crystal will be peeled off due to shear and fracture, which produces a part at which the first zeolite crystal is exposed, resulting in a decreased molecular sieving effect.
In addition, Patent Document 2 listed below discloses a method for coating solid acid catalyst particles with zeolite crystals having a molecular sieving effect. According to this method, however, each catalyst particle is relatively large with an average particle size of 0.3-3.0 mm and has a thick coating layer with a thickness of 1-100 μm. Therefore, it is believed that a body to be treated, such as the raw material and product, experiences a large resistance when passing through a silicate film, which results in an insufficient reaction efficiency, low conversion of toluene and significantly low yield of paraxylene. On the other hand, if the thickness of the coating film is reduced, the coating may be damaged due to physical damage, etc.
Furthermore, Patent Document 3 listed below discloses a catalyst that comprises a core made of crystalline borosilicate and a shell made of silicon oxide (crystalline silicate) having the same crystalline structure as the core. However, this catalyst indeed defines a weight ratio of shell/core in relation to the crystalline silicate of the shell, but there is no reference to the thickness, uniformity, defects or the like of silicate which determines reaction results. Additionally, no definition is provided as to the particle size or crystallite size for the crystalline borosilicate of the core. Such a catalyst would bring about incomplete formation of a silicate coating film, thereby significantly decreasing reaction activity. Otherwise, since a part of the outer surface of the core zeolite is exposed, it is difficult to achieve a reaction with highly controlled selectivity as is the case for obtaining high purity paraxylene.
In addition, Patent Document 4 listed below discloses a catalyst that is formed by coating MFI type zeolites having a particle size of not more than 100 μm with a crystalline silicate, and teaches that the catalyst coated with the crystalline silicate exhibits extremely high paraxylene selectivity and that a para-substituted aromatic hydrocarbon may be produced in an efficient manner as compared with the prior art. However, the reaction rate under the conditions described in Patent Document 4 is not adequate for commercialization. It is desired to develop a catalyst that has a higher reaction rate.